Description of a conventional balun will be given in the following referring to FIG. 4, FIG. 5, and FIG. 6.
FIG. 4 is a circuit diagram of a conventional balun. In FIG. 4, conventional balun 1 comprises input terminal 2, coupled line 3 connected to input terminal 2, and first output terminal 4 and second output terminal 5 connected to coupled line 3.
Additionally, first capacitor 6 is connected between input terminal 2 and coupled line 3 in shunt with ground. Also, the lines from coupled lines 3 to first output terminal 4 and from coupled line 3 to second output terminal 5 are RF-coupled through second capacitor 7.
Also, coupled line 3 includes first unbalanced line 8 connected to input terminal 2, and second unbalanced line 9 connected between first unbalanced line 8 and ground. Furthermore, coupled line 3 includes first balanced line 10 connected between ground and first output terminal 4 and electromagnetically coupled to first unbalanced line 8, and second balanced line 11 connected between ground and second output terminal 5 and electromagnetically coupled to second unbalanced line 9.
FIG. 5 is a sectional view of a conventional balun. In FIG. 5, conventional balun 1 includes first dielectric layer 12 and second dielectric layer 13 on top of first dielectric layer 12. First unbalanced line 8 and second unbalanced line 9 are disposed on the same plane on top of second dielectric layer 13. Furthermore, first balanced line 10 and second balanced line 11 are disposed on the same plane on top of first dielectric layer 12 under first unbalanced line 8 and second unbalanced line 9, respectively, through second dielectric layer 13.
FIG. 6 is a perspective view of a coupled line of a conventional balun. As can be seen in FIG. 6, first unbalanced line 8 and second unbalanced line 9 are connected by folding back. Size reduction of balun 1 is achieved with this design.
As a related art document relating to the invention of this application, Patent Document 1 is known, for example:
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-190413.
However, as first unbalanced line 8 and second unbalanced line 9 are connected by folding back, first unbalanced line 8 and second unbalanced line 9 come close to each other. As a result, the current of a radio frequency signal propagating in first unbalanced line 8 and the current of a radio frequency signal propagating in second unbalanced line 9 cancel each other thus resulting in attenuation of balanced signals obtained from first output terminal 4 and second output terminal 5 of balun 1 when compared with an unbalanced signal inputted to input terminal 2 of conventional balun 1.